import React,  { Component } from 'react';

export default class Count extends Component {
  state = { count: 0};

  // 加法
  increment = () => {
    const { value } = this.selectNumber
    const { count } = this.state
    this.setState({ count: count + value*1 })
  }

  // 减法
  decrement = () => {
    const { value } = this.selectNumber
    const { count } = this.state
    this.setState({ count: count - value*1 })
  }

  // 奇数再加
  incrementIfOdd = () => {
    const { value } = this.selectNumber
    const { count } = this.state
    if (count %2 !== 0) {
      this.setState({ count: count + value*1 })
    }
  }

  // 异步加
  incrementAsync = () => {
    const { value } = this.selectNumber
    const { count } = this.state
    setTimeout(() => {
      this.setState({ count: count + value*1 })
    }, 500);
  }

  render () {
    return (
      <div>
        <h1>当前求和为：{this.state.count}</h1>
        <select ref={e => this.selectNumber = e}>
          <option value="1">1</option>
          <option value="2">2</option>
          <option value="3">3</option>
        </select>&nbsp;
        <button onClick={this.increment}>+</button>&nbsp;
        <button onClick={this.decrement}>-</button>&nbsp;
        <button onClick={this.incrementIfOdd}>当前求和为奇数再加</button>&nbsp;
        <button onClick={this.incrementAsync}>异步加</button>&nbsp;
      </div>
    )
  }
  
}